The present invention relates to twisted pair cables which can be used in high frequency applications.
Twisted pair cables have become the physical media of choice for local area networks in the last 10 years. The current EIA/TIA 568 A Category 5 specifications (and the associated addenda) for these cables call for performance up to a frequency of 100 MHz.
Installed transmission systems were, until recently, operating only at 10 Mbit/s and did not use all the available bandwidth offered by cables meeting the existing specifications. In fact, the Ethernet protocol used in over 70% of the installed networks, employs only two pairs of the available four and uses half-duplex transmission, i.e. one pair is transmitting while the other is receiving.
In the last five years, new transmission technology, operating at 100 Mbit/s has been rapidly expanding in the marketplace. At the same time, improved cables with transmission characteristics exceeding the current EIA/TIA 568 A Category 5 specifications (and the associated addenda) were also developed. Despite the assurance of performance promised by the existing specifications, cable manufacturers have developed cables with improved performance as an insurance policy for future applications. In addition, process variation during the manufacture of the cable and further handling during installation were causing deterioration in cable performance, thus the requirement of transmission characteristics that exceeded the current specifications.
More recently, new data transmission technology has indeed pushed the speed limit to 1 Gigabit/s and higher. This transmission technology and some of the existing 100 Mbit/s transmission technologies, when applied to twisted pair cables, require the use of all four pairs in a cable in duplex operation (bi-directional transmission). These new protocols have increased noticeably the transmission performance requirements of the twisted pair wire cables beyond the EIA/TIA 568 A Category 5 specifications (and the associated addenda).
In the first place, the delay skew or the differential in the signal velocity amongst the 4 pairs has to be minimal in order to enable fast de-scrambling of the four bit signals into a coherent bit sequence at the receiving end.
Additional capabilities for bi-directional transmission are also required in order to obtain the maximum bandwidth available on a 4-pair twisted cable. This last requirement introduces the possibility of multi-pair power sum near end, equal level far end and multi-pair power sum equal level far end cross-walk, as well as the increased possibility that return loss (due to impedance irregularities) will impair transmission. Twisted pair cables have to be designed with low and uniform near and far end cross-talk and, consequently, low power sum cross-talk, equal level (less the attenuation) far end and power sum equal level far end cross-talk.
Recent Category 5E addenda to the EIA/TIA 568 A specifications has taken into account these new requirements. However, there is no consensus yet on the specifications for a twisted pair cable that will meet the requirements for beyond 1 Gbit/s transmission. The first draft C1 for such a new specification introduces the new Category 6 cabling system and has its ISO counterpart draft specification (ISO/IEC SC25 WG3 Proposal).
There are already in the marketplace several cable designs that claim to meet and even exceed the proposed Category 6 specifications. The first cable design that claims gigabit capability was developed by Belden Wire and Cable Company (U.S. Pat. No. 5,606,151 to Siekierka et al.) and uses the joining of the two insulated conductors in a pair by means of an adhesive or by co-extruding the two insulated conductors with a very small joining web. This device is meant to mainly improve the longitudinal impedance uniformity to less than +/xe2x88x9215 ohm and, as a result, to minimize return loss impairments of the resulting 4 pair twisted cable. The claimed reason for the observed reduction in impedance irregularities is explained by the fact that cyclical and random irregularities that can be imparted in the twisted pair during the twisting process due to differences in twisting tension are eliminated when the bonded pairs are twisted together. It is also claimed that the cable resists deformation during process handling and installation.
In addition, the cable described in this patent uses a crescent cable structure whereby each pair is secured in a single tube-like slot. The manufacturer claims improved near end and far end cross-talk performance for this design. However, this structure is exceedingly difficult to manufacture as each tube-like slot cannot have even the smallest variations in diameter without a marked deterioration of the electrical characteristics. When cables are stacked together in installations, there are also greater chances for inter cable cross-talk impairments due to the proximity of pairs with same twisting lays separated only by the jacket thickness. The bonded pairs are also difficult to strip and install. This design does not impart any additional advantage as far as the reduction of cross-talk impairments is concerned. It also does not eliminate impedance variations that can be caused by off centre, oval or otherwise irregularly shaped insulation.
U.S. Pat. No. 5,767,441 to Brorein et al. claims to eliminate such impedance variations through the pre-twisting of insulated conductors prior to twisting the insulated conductors in double twist machines or by twisting the pairs through a single twist process. This process has unleashed a flood of equipment designed to impart back-twist capabilities for manufacturers of high performance cables. In addition, this patent discloses a flat cable structure, similar to the cable described in the previous patent. The manufacturing process of this cable is also prone to cause small variations in the pair slot dimensions, thus compromising the transmission performance of the resulting 4-pair cable. In addition, the structure of these flat cable designs may pose additional transmission problems, due to inter-cable cross-talk or xe2x80x9calien cross-talkxe2x80x9d that cannot be cancelled electronically through DSP filtering.
Another solution to gigabit performance requirements has been put forth by the proponents of cables with central members whereby the twisted pairs are separated by means of a longitudinal central member (CommScope Isolator(trademark) design, Hitachi Manchester""s HI-NET(trademark) and other designs). This design affords the greatest reduction of cross-talk impairments but does not eliminate impedance irregularities. The insertion of a central member with the four pairs symmetrically disposed around it is difficult to achieve and slows down the manufacturing processes. In addition, the cable diameter is increased by at least 20%. The overall cost of the cable is also substantially increased due to the additional cost of the center member and higher jacketing material costs.
It is the object of this invention to eliminate many of the difficulties inherent in the cables of the prior art while substantially reducing both cross-talk impairments and impedance irregularities in a cost competitive manner respectful of the EIA/TIA specifications.
In accordance with the invention, this object is achieved with a twisted pair cable comprising a plurality of pairs, each of said pairs comprising two conductors, each of said conductors is covered with an inner layer insulator and an outer layer insulator, said conductors being eccentric with respect to the overall insulation of said inner and outer layer insulator.
The present invention also concerns a method for making the same.